"See you later!"
The added 'x' is a typing shorthand or code. So you don't have to have a special font or character keyboard. However, since Duolingo has the character inputs (buttons below the input box), they likely don't use this. And, imo, it's better to learn the actual character. A bit like the "imo" I just used, it's fine for casual internet use, but otherwise it's not really used. ;)